Electrophotography, as disclosed in U.S. Pat. No. 2,297,691 to Carlson, uses a photoconductive material comprising a support coated with a substance which is insulating in the dark, and having an electric resistance which changes according to the amount of exposure received during imagewise exposure. After this photoconductive material is subjectd to keep in dark for a suitable period, the surface is uniformly charged in the dark. Then, this material is imagewise exposed to light using an irradiation pattern having the effect of reducing the surface electric charge in a pattern corresponding to the relative energy contained in various parts of the irradiation pattern. The surface electric charge or electrostatic latent image left on the surface of the photoconductive substance layer (electrophotographic light-sensitive light) is then contacted with a suitable electroscopic indicating substance, e.g., toner, to form a visible image.
The toner is contained in an insulating liquid or a dried carrier, and in either case it adheres to the surface of the electrophotographic light-sensitive layer according to the electric charge pattern. The adhering indicating substance can be fixed by known means such as by heat, pressure or vapor of a solvent. The electrostatic latent image can be transferred to a second support (for example, paper or films). The electrostatic latent image can be developed on the second support in a similar fashion after being transferred. Electrophotography is one image forming process in which the image is formed as described above.
In such an electrophotographic process, the basic characteristics required for electrophotographic light-sensitive materials are that (1) they can be electrified in the dark so as to have a suitable electric potential, (2) the disappearance of electric charge in the dark is small, and (3) the electric charge can be rapidly reduced by exposure to light.
Conventionally as photoconductive substances for electrophotographic light-sensitive materials use has been made of selenium, cadmium sulfide, zinc oxide, etc.
However, the above described inorganic substances conventionally used in electrophotographic processes have both faults and advantages. For example, selenium which is widely used at present satisfies the above described requirements (1) to (3), but has the disadvantages that its production requires severe conditions and high production cost, it is inflexible and difficult to process into a belt-like form, and it requires careful handling because it is sensitive to heat and mechanical impact. Cadmium sulfide and zinc oxide are used as electrophotographic light-sensitive materials dispersed in a resin which is a binder. However, they cannot be repeatedly used in such a state, because of mechanical deficiencies in smoothness, hardness, tensile strength and friction resistance.
In recent years, to improve these faults of inorganic substances, electrophotographic light-sensitive materials using various organic substances have been proposed and some of them have been put in practical use. Examples include electrophotographic light-sensitive materials containing poly-N-vinylcarbazole and 2,4,7-trinitrofluoren-9-one (U.S. Pat. No. 3,484,237), or poly-N-vinylcarbazole sensitized with pyrylium salt type dyes [Japanese Patent Publication No. 25658/73 (U.S. Pat. No. 3,617,268)], electrophotographic light-sensitive materials containing an organic pigment as a main component [Japanese Patent Application (OPI) No. 37543/72 (U.S. Pat. No. 3,898,084)] and electrophotographic light-sensitive materials containing an eutectic crystal complex composed of a dye and a resin as a main component [Japanese Patent Application (OPI) No. 10735/72 (U.S. Pat. Nos. 3,732,180 and 3,684,502)] (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Electrophotographic light-sensitive materials using such organic materials are high in productivity, can provide inexpensive light-sensitive materials, and can be improved in mechanical properties and flexibility since they can be produced by coating by choosing a binder suitably. In addition, the light-sensitive wavelength can be arbitrarily controlled by choosing a dye and an organic pigment. However, the light-sensitivity is low and it is not suitable to be used repeatedly, which does not satisfy well enough the requirement for an electrophotographic light-sensitive material.
As the results of thorough studies by the inventors to overcome these faults of the conventional electrophotographic light-sensitive materials, it has been found that an electrophotographic light-sensitive material using a photoconductive composition containing a novel tetrakisazo compound has high sensitivity and high durability (repeated use) enough to be used in practice.
On the other hand, since the conventional recording method wherein a beam having a high energy density is irradiated on an information recording medium to change the physical constants, such as transmittance, reflectance, refractive index, etc., is characterized in that it can form an image having contrast with a quite high resolving power, information can be added afterward, recording can be effected simultaneously with exposure to light, and that it has other characteristics, it has advantages in that it is suitable for recording an output of an electronic computer and transmitted time series signals, and is used for COM (computer output micro), microfacsimiles, printing masters, optical disks, etc.
For example, the recording medium used in optical disk technique includes fine pits on the order of about 1.mu. that can be detected optically and are arranged in the form of a helical track or circular tracks and can store high density information. To write information onto such a disk, a laser beam converged on the surface of a laser beam-sensitive layer is scanned so that pits may be formed only in the surface irradiated with the laser beam, which pits should be formed in the form of a helical track or circular tracks. In the recording system of heat mode, a laser beam-sensitive layer absorbs the heat energy when irradiated with a laser beam whereby the concave part is formed with pits due to evaporation or melting.
The information thus recorded in an optical disk can be detected by scanning a laser beam along the track to read the optical change caused by the presence or absence of the pits.
As such an information recording medium capable of recording by heat mode, use has been made a recording medium wherein a transparent support of plastics or the like has thereon a thin film of a metal and/or a metal oxide metalloid dielectric, etc. or a thin film containing a self-oxidizing binder and a dye and a protective layer thereon.
However, since the conventional thin film mainly consisting of inorganic materials is high in reflectance with respect to laser beam, the efficiency of the use of the laser beam is low, and therefore high sensitivity characteristic cannot be obtained or the output of the laser beam when recording is effected must be increased remarkably.
On the other hand, the nearer the longer wave length region, generally the more unstable the absorption characteristic of organic compounds, so that a little elevation of temperature is liable to decompose them.
Therefore, it is required that the absorption efficiency of the laser beam required for a recording medium having capability of direct reading and writing is high, an enough reflectance is secured to be able to control the focus when reading of the record is carried out, and various characteristics such as stability of the recorded image must be satisfied. In this regard, a recording medium containing an organic thin film satisfactory enough from practical points of view has not yet been developed.
In view of the above points, to overcome the above faults the inventors have intensively studied to find the present invention.